1. Field of the Invention
This invention relates to circuitry for driving an inductively loaded switching transistor, and more particularly to such circuitry which employs a bootstrap circuit to couple the load with the transistor drive.
2. Description of the Prior Art
Distinct problems can arise when power transistors are used to switch relatively high levels of current. For example, in recent years the use of offline switching power supplies has become popular. These devices are run directly off a main power line, say 115 volts, and require that the transistors employed in switching the inductive inverters be capable of switching currents in the order of 10 to 15 amperes. The problem is compounded because, in order to accommodate the high voltages involved, transistor gain (beta) is generally sacrificed to the extent that betas of 3 to 5 are typical. Thus, the transistor drive circuits must be capable of supplying up to 5 amperes to the base of each switching transistor.
The basic configuration of a typical drive circuit of conventional design is illustrated in FIG. 1. In this circuit a switching transistor Qa controls the application of power to an inductive load 1, such as one leg of an inverter. Qa is actuated to a conductive state by the application of an appropriate signal to the base of an actuating transistor Qb, which becomes conductive and permits current flow from a positive bus V.sup.+, through a current limiting resistor Ra, to the base of Qa. Ra must be small enough to permit the high base currents referred to above under heavy load conditions, and also to allow for manufacturing tolerances in the transistors. As a result the current delivered to the base of Qa may be considerably greater than that required to keep the transistor conductive, especially during the initial transient period before full load develops and during periods of relatively low loads. This, in turn, leads to an unnecessary power loss which reduces the efficiency of the power supply, and in addition switching transistor Qa may become saturated and thereby suffer a deterioration in its turn-off time. At the end of the driving period, the drive signal is removed from the base of Qb and a separate signal is applied to gate a third transistor Qc, which connects the base of Qa to ground to enhance turn-off.
It can thus be seen that the conventional drive circuit described above suffers in terms of both power efficiency and the time required for turn-off. Various bootstrap circuits have been developed which could be used to alleviate the efficiency problem by continuously tying the magnitude of the gating current supplied to the switching transistor to the magnitude of the load current. Thus, a decrease in load current is accompanied by a decrease in the base current necessary to keep the switching transistors conductive.
While greatly enhancing the efficiency of the drive circuit, the basic bootstrap does not solve all the problems, and introduces some of its own. It does not provide a means for initially gating the switching transistor, and should the load current subsequently dip low enough it is possible for the transistor to turn off with no means to re-actuate it. There may still be a problem in the time required to turn off the switching transistor, which may be aggravated if the load current preceding turn-off goes high enough to drive the transistor into saturation. Furthermore, there may still be a need for a separately controlled turn-off transistor.